Opal glasses have been known to the glass art for many years. Such glasses contain a second phase dispersed therewithin which exhibits a refractive index which differs from that of the glassy matrix. It is this difference in refractive index which imparts opacification to the glass body. Opal glasses have been utilized extensively in such consumer product applications as culinary ware and tableware.
There are two broad types of opal glasses based upon the manner of manufacture thereof. The first group has been termed spontaneous opacifiable glasses or "spontaneous opals". Such glasses develop opacity as the molten batch is cooled and shaped into a glass article. The second group has been termed thermally opacifiable glasses or "reheat opals". Those glasses require the application of a subsequent heat treatment to a glass body to effect opacification therein. Since the cost of manufacture is a significant factor in determining the selection of an opal glass for a particular utility, it is apparent that "spontaneous opals" are preferred to "reheat opals".
The opacity observed in "spontaneous opals" is commonly generated through the growth of crystals therein as the molten batch is cooled and shaped into a glass article of a desired configuration. Customarily, the crystal content developed in situ is less than 10% by volume of the body such that the physical properties of the glass, other than optical transmission, will be very slightly affected, if at all.
Whereas opal glasses have been and are presently marketed in the as-formed state, i.e., the undecorated state, consumer interest has greatly increased for articles having decorations thereon. For example, in culinary ware and tableware, the consumer has demanded products having an aesthetically pleasing appearance coupled with long time service capability. Thus, such products are subject to chemical and mechanical abuse resulting from handling, food contact, and, particularly, from contact with dishwasher detergents.
The tinting of glass to impart an integral color thereto has been practiced in the art for a long time. Coloured Glasses, W. A. Weyl, Dawson's of Pall Mall, London, 1959, is a classic text which surveys the wide variety of ions which can be used to impart colors to glass and suggests mechanisms by which such colors are generated. Tinting glass has the obvious advantage in that the color exists throughout the body of the article and will not be removed via surface abuse.
Spontaneously opacifiable glass bodies wherein alkali metal fluoride and/or alkaline earth metal fluoride crystals comprise the opacifying phase are well-recognized in the glass art. Such bodies exhibit a dense, milky white appearance and have been employed extensively in the formation of culinary ware and tableware.